A method capable of forming an image of superhigh contrast (particularly having a gamma value (.gamma.) of at least 10) is needed in the graphic art field in order to ensure good reproduction of an image of continuous tone utilizing a dot image or good reproduction of line image.
Heretofore, for this purpose, a special developer called a "lith developer" has been used. The lith developer contains only hydroquinone as a developing agent, and the concentration of free sulfite ions in the lith developer is greatly decreased usually to not more than 0.1 mol/l by adding a sulfite as a preservative in the form of an adduct with formaldehyde so as not to inhibit the infectious developing properties of the lith developer. For this reason, the lith developer has a vital disadvantage in that it is readily subject to air oxidation and thus cannot be stored for more than 3 days.
In order to obtain an image of high contrast using a stable developer, U.S. Pat. Nos. 4,224,401, 4,168,977, 4,166,742, 4,311,781, 4,272,606, 4,211,857 and 4,243,739 disclose methods using hydrazine derivatives. In accordance with these methods, photographic characteristics of superhigh contrast and high sensitivity can be obtained, and moreover, since a high concentration of a sulfite can be added to the developer, the stability of the developer against air oxidation is markedly increased compared with the lith developer.
The above image forming system, however, causes an undesirable phenomenon, e.g., formation of black spots due to infectious development although it provides high sensitivity and high contrast. This formation of black spots is a serious problem in the photomechanical process. These black spots, sometimes called "black pepper", appear at positions between halftone dot and halftone dot, which are to become non-exposed areas. The formation of black spots is accelerated when the light-sensitive material is stored for long periods of time, particularly under high temperature/high humidity conditions, or by a reduction in the amount of sulfite ions generally used as a preservative and an increase in pH as a result of fatigue of the developer with a lapse of time. Thus, the commercial value of the light-sensitive material as a material for use in the photomechanical process is seriously decreased.
In order to overcome the problem of black spots as described above, extensive investigations have been made. Improvement of black spots, however, often leads to a reduction in sensitivity and gamma (.gamma.). It has therefore been desired to overcome the problem of black spots while maintaining sensitivity and contrast at high levels.